It is known that modulational instabilities, though typically requiring negative GVD to grow, can experience gain if higher order dispersion parameters are negative [1]}. At 10.6 \(\mu \) m \(\beta _m < 0\)  for \(m\ge 4\) , and using a perturbative expansion of the instability gain up to \(m=8\)  [2]}, gain only exists at wavelengths between 8.5 and 9.6 \(\mu \) m. Downshifting by these frequencies cannot occur, and no evidence of high-frequency sidebands was found. Thus the observed SC generation is likely caused by SRS in combination with SPM, supported by the relatively strong sideband efficiency.
